Though should we file a separate issue for the initial slash thing, as that probably isn't a quick fix -- we'd need input from routing system developers to know whether it's expected behaviour or a glitch that it works without the '/', and then docs would need updating etc etc.

The test block appears in the custom category. Other DisplayBlockTest.php 102 Drupal\block\Tests\Views\DisplayBlockTest->testBlockCategory()
The second cloned test block appears in the custom category. Other DisplayBlockTest.php 120 Drupal\block\Tests\Views\DisplayBlockTest->testBlockCategory()